This invention relates generally to rolling loop film transport mechanisms of the type used to transport photographic film through a projector, camera or other motion picture apparatus and more particularly to a shock absorbing film guide mechanism for prolonging the life of films employed in such mechanisms.
U.S. Pat. No. 4,114,996 describes a film inlet guide having fixed complementary outer and inner guides adapted to receive film from a drive sprocket of a rolling loop film transport mechanism and to guide the film inwardly toward a path defined between the stator and rotor of the film transport mechanism. This prior art film inlet guide is disadvantageous in several respects. Excessive film wear results from the friction created by the film passing the stationary surfaces of the inlet guide, particularly the nose portion of the outer guide. Over time, this friction produces wear on the nose portion of the outer guide itself, requiring periodic film guide replacement. In addition, a static charge is created by the film passing over the stationary nose portion of the outer guide. This static charge attracts dust and other particles to the film surface immediately prior to projection onto a screen, resulting in magnification of these particles to a size 350 times their actual size. The nose portion of the outer guide of the prior art film inlet guide causes the film to become rigid as it enters the gap on the rotor of the film transport mechanism. When the rigid length of film is impacted by the rotor segments that are travelling at 180 RPM, the film is lightly tugged against the teeth of the inlet drive sprocket, thereby causing film wear and/or damage to the perforations along the edges of the film. The rigidity imparted to the film by this prior art inlet film guide becomes an even more significant problem when film speed is increased from the conventional 24 frames per second to the 30 frames per second at which some projection systems operate. Finally, the film inlet guide of the prior art hinges to an unadjustable indexed position that defines a fixed gap between the inlet sprocket and the film guide. Since this gap is fixed, any backlash or shock rebounds of the film within the gap results in additional abrasion of the film surface.
It is therefore the principal object of the present invention to provide a film guide mechanism for use in rolling loop film transport mechanisms that minimizes film wear and therefore prolongs the life of motion picture film operating at a conventional speed of 24 frames per second and that facilitates higher speeds on the order of 30 frames per second that were previously not attainable using prior art film guide mechanisms.
It is a further object of the present invention to provide a film guide mechanism for use in rolling loop film transport mechanisms that significantly reduces the static charge buildup on the film, resulting in a much cleaner presentation of the film as it is projected onto a large screen.
It is a further object of the present invention to provide a film guide mechanism for use in rolling loop film transport mechanisms that prevents rigidity of the film as it exits the film guide mechanism, thereby reducing the force with which the film impacts the rotor and, as a result, minimizing wear on the film and the film guide mechanism itself.
It is a further object of the present invention to provide a film guide mechanism for use in rolling loop film transport mechanisms that permits easy adjustment, within a particular range, of a film exit gap between a shock double capstan and a fixed inner film guide.
It is a further object of the present invention to provide a film guide mechanism for use in rolling loop film transport mechanisms that includes a shock absorber that permits variability of the film exit gap during operation in response to backlash and rebounding of the film as it passes through the film exit gap, thereby eliminating stress on the film perforations imparted by the teeth of the driven inlet drive sprocket.
These and other objects are accomplished in accordance with the illustrated preferred embodiment of the present invention by providing a driven inlet drive sprocket having a plurality of teeth that mate with perforations along each edge of the film and that also mate with grooves in a shock double capstan that is in turn driven by the driven inlet drive sprocket, thereby sandwiching the film therebetween. The driven inlet drive sprocket and the shock double capstan are slightly concave in shape so that no contact is made between these components and the emulsion area of the film. A fixed inner guide has a radius in an area adjacent. The shock double capstan that matches the radius of the shock double capstan itself to provide a variable gap to accommodate any backlash or shock rebound of the film as it passes through the gap. The gap is made variable by a shock absorber assembly coupling the shock double capstan to an adjusting screw via a spring. The spring constant of the spring is set by means of the adjusting screw, thereby allowing the gap to vary in response to any shock rebounds of the film that may occur within the gap.